1. Field of the Invention
The present invention relates to a particle monitoring apparatus configured to be arranged in a so-called clean room or the like, mainly in a vacuum processing apparatus in a semiconductor producing process line or the like and to monitor sizes and number of particles (“dust” so-called particles, etc.) in a gas stream. The invention also relates to a vacuum processing apparatus equipped with the particle monitoring apparatus. More particularly, the invention relates to an improvement on the gas-bonre particle monitoring apparatus configured to determine the sizes of the particles depending upon the intensities of the lights scattered by the particles.
2. Related Art Statement
In the semiconductor producing process line, for example, particles are heretofore removed to a high level, while the number and sizes of the remaining particles are constantly being monitored.
A particle counter (particle monitoring apparatus) has been known to monitor such particles (contamination/supernatant). The particle counter is arranged inside an exhausting section of a processing apparatus in the semiconductor producing process line, for example.
When the gas stream in the process, which flows into the exhausting section, passes a light beam (light flux) shaped in a sheet-like (band-like) form, any particles contained in the gas stream scatter the light. Therefore, presence of the particles can be detected by sensing the scattered light (Patent literature 1: JP-A 2000-146819).
It is known that there is a specific correlation between the intensity of the detected scattered light and the size of the particle. Thus, if the correlation between the sizes of the particles and the intensities of the scattered lights is empirically determined in advance, the sizes of the particles can be determined based on the detected intensities of the scattered lights in actual use.
Further, the particle counter monitors the cleanness state inside the dry etching apparatus by countering the number of particles for each of the discriminated sizes of the particles.
When the correlation between the sizes of the particles and the intensities of the scattered lights is to be determined in advance, test particles having known sizes (PSL: Polystyrene Latex) are used.
On the other hand, it is laterally inhomogeneous that the projected light intensity distribution of the band-shaped light beam formed with the conventional particle counter. The band shape of the projected light intensity distribution in the horizontal direction of the laser beam is Gauss distribution. And, it is identified not a function mode (TEM00: Transverse Electromagnetic 00) but a multimode in the propagating characteristic that Gauss distribution of a light induced through a multimode-fiber with a projected light system.
According to any conventional particle counter, the widthwise central portion of the light beam has the highest light intensity, and the light intensity decreases as the location goes from the center toward a widthwise edge portions of the light beam.
Therefore, the intensity of the scattered light obtained when the particle passes the widthwise central portion of the band-shaped light beam is different from that of the scattered light obtained when the particle passes near the widthwise edge portion of the band-shaped light beam, even if the size of the former particle is identical with that of the latter one. The intensity of the scattered light obtained when the particle passes the widthwise central portion of the band-shaped light beam is greater than that of the scattered light obtained when the particle passes near the widthwise edge portion of the band-shaped light beam.
For this reason, when the size of the particle is determined based on the detected intensity of the scattered light, it is feared that a considerable error is contained in the determined value.
When the correlation between the sizes of the particles and the detected intensities of the scattered lights is to be determined in advance, the intensities of the scattered lights are repeatedly detected by using the test particles having the same size, or the intensities of the scattered lights are detected by using a number of the test particles determined corresponding to said size based on the frequency distribution of the detected intensities of the scattered lights. However, since the intensity of the scattered light depends upon the passing position of the particle through the light beam as mentioned above, a peak among the frequencies is not clear. Consequently, there is a problem in that the correlation between the sizes of the particles and the intensities of the scattered lights cannot be accurately set.